Industrial vinegar production is usually conducted by means of microbial fermentation. Acetic acid bacteria, in particular, contribute to the generation of acetic acid in vinegar fermentation. Acetic acid bacteria were classified into two genera, Acetobacter and Gluconobacter, depending upon their abilities to over-oxidize acetate or lactate and the positions of their flagella (De Ley et al., Family VI. Acetobacteraceae. In Bergey's Manual of Systematic Bacteriology Vol. 1, pp. 267-278. Baltimore: Williams & Wilkins, 1984). Since Acetobacter prefers oxidizing ethanol instead of glucose for producing acetic acid but Gluconobacter prefers to the contrary, most strains useful in vinegar manufacture belong to Acetobacter, whereas Gluconobacter is used for industrial applications, such as fermentation of ketoglucomic acid, sorbose and dihydroxyacetone (Swings, The genera Acetobacter and Gluconobacter. In The Prokaryotes Vol. III, pp. 2268-2286, New York: Springer Verlag, 1991). Among the Acetobacter species, Acetobacter aceti, Acetobacter pasteurianus, Acetobacter polyoxogenes and Acetobacter europaneus are the most popular strains for making acetic acid in vinegar factories on the grounds that their oxidization of ethanol is superior and they hardly destroy acetic acid later (U.S. Pat. No. 4,654,306; Sievers et al., Systematic Applied Microbiology 15, 386-392, 1992).
Most bacteria useful for acetic acid production are mesophilic strains with optimum temperature for growth at about 30.degree. C. These strains are neither capable of growing at temperatures above 30.degree. C. nor producing acetic acid when they are cultured at temperatures higher than 32.degree. C. (Nalayama, Journal of Biochemistry 49, 158-163, 1961). This sensitivity to elevated temperature brings a limitation to most industrial vinegar production which is not strictly controlled at 30.degree. C. In recent years, the temperature in summer has gradually elevated in many tropical and subtropical countries. As an example in Taiwan, the average indoor temperature at nights during the summer is above 30.degree. C. Therefore, a majority of local vinegar manufacture is limited to a great extent for almost half a year. As far as those critical factors of vinegar production are concerned, an ideal bacterial strain should possess high efficiency of acetic acid production as well as remarkable tolerance to intermediates and products in considerable amounts which are generated during fermentation, such as ethanol and acetic acid.
Some efforts have been made to improve the bacterial properties which are advantageous in vinegar industry, such as increased acetic acid production rate and enhanced tolerance to high acetic acid, ethanol and fermentation temperature (Lotong el al., Applied Microbiology and Biotechnology 32, 27-31, 1989).
However, there is still a need in the vinegar industry to identify a new acetic acid-producing strain having versatile advantages, including thermotolerance, resistance to ethanol and high acetic acid productivity. In particular for vinegar industry in tropical and subtropical areas, there is still a need of a thermotolerant strain capable of propagating and producing acetic acid at a temperature above 30.degree. C.